Light is electromagnetic radiation that is visible to the human eye. The frequency range of light is:
- Infrared light—less than 400,000 gigahertz
- Visible light—400,000 to 750,000 gigahertz
- Ultraviolet light—greater than 750,000 gigahertz
Light waves at the upper end of the frequency range have more energy than light waves at the lower end.
Light Sensitive Devices
Semiconductor devices that interact with light can be classified as: light-detection devices, light-conversion devices, or light-emitting devices.
Light-sensitive devices include photo cells, solar cells, photodiodes, and phototransistors. Light-emitting devices include the LED (light-emitting diode).
The photoconductive cell (photo cell) is the oldest opto-electric device. It is a light-sensitive device in which the internal resistance changes with a change in light intensity. The resistance change is not proportional to the light striking it. Figure 1 shows a typical photo cell.
The photo cell is made from light-sensitive material such as cadmium sulfide (CdS) or cadmium selenide (CdSe). The light-sensitive material is deposited on an insulating substrate of glass or ceramic in an S-shape to allow greater contact length.
The photo cell is more sensitive to light than any other device. The resistance can vary from several hundred million ohms to several hundred ohms.
The photo cell is useful for low-light applications. It can stand high operating voltages of 200 to 300 volts with a low power consumption of up to 300 milliwatts. A disadvantage of the photo cell is its slow response to light change.
Figure 2 shows the schematic symbols used to represent a photo cell. The arrows indicate a light-sensitive device. Sometimes the Greek letter lambda (λ) is used to represent a light-sensitive device.
Photo cells are used in light meters for photographic equipment, instruction detectors, automatic door openers, and various kinds of test equipment to measure light intensity.
Solar Cell
The photovoltaic cell (solar cell) converts light energy directly into electrical energy. The solar cell finds most of its applications in converting solar energy into electrical energy.
The solar cell is basically a PN junction device made from semiconductor materials. It is most commonly made from silicon. Figure 3 shows the construction technique.
The P and N layers form a PN junction. The metal support and the metal contact act as the electrical contacts. It is designed with a large surface area. The light striking the surface of the solar cell imparts much of its energy to the atoms in the semiconductor material.
The light energy knocks valence electrons from their orbits, creating free electrons. The electrons near the depletion region are drawn to the N-type material, producing a small voltage across the PN junction. The voltage increases with an increase in light intensity.
All the light energy striking the solar cell does not create free electrons, however. In fact, the solar cell is a highly inefficient device, with a top efficiency of 15 to 20%. The cell is inefficient when expressed in terms of electrical power output compared to the total power contained in the input light energy.
Solar cells have a low voltage output of about 0.45 volt at 50 milli-amperes. They must be connected in a series and parallel network to obtain the desired voltage and current output.
Applications include light meters for photographic equipment, motion-picture projector soundtrack decoders, and battery chargers on satellites.
Schematic symbols for the solar cell are shown in Figure 4. The positive terminal is identified with a plus (+) sign.
Photodiode
The photodiode also uses a PN junction and has a construction similar to that of the solar cell. It is used the same way as the photo cell, as a light variable resistor. Photodiodes are semiconductor devices that are made primarily from silicon.
They are constructed in two ways. One method is as a simple PN junction (Figure 5). The other method inserts an intrinsic (undoped) layer between the P and N region (Figure 6), forming a PIN photodiode.
A PN junction photodiode operates on the same principles as a photovoltaic cell except that it is used to control current flow, not generate it.
A reverse-bias voltage is applied across the photodiode, forming a wide depletion region. When light energy strikes the photodiode, it enters the depletion region, creating free electrons. The electrons are drawn toward the positive side of the bias source. A small current flows through the photodiode in the reverse direction. As the light energy increases, more free electrons are generated, creating more current flow.
A PIN photodiode has an intrinsic layer between the P and N regions. This effectively extends the depletion region. The wider depletion region allows the PIN photodiode to respond to lower light frequencies.
The lower light frequencies have less energy and so must penetrate deeper into the depletion region before generating free electrons. The wider depletion region offers a greater chance of free electrons being generated.
The PIN photodiode is more efficient over a wide range. The PIN photodiode has a lower internal capacitance because of the intrinsic layer. This allows for a faster response to changes in light intensity. Also, a more linear change in reverse current with light intensity is produced.
The advantage of the photodiode is fast response to light changes, the fastest of any photosensitive device. The disadvantage is a low output compared to other photosensitive devices.
Figure 7 shows a typical photodiode package. A glass window allows light energy to strike the photodiode. The schematic symbol is shown in Figure 8. A typical circuit is shown in Figure 9.
Phototransistor
A phototransistor is constructed like other transistors with two PN junctions. It resembles a standard NPN transistor. It is used like a photodiode and packaged like a photodiode, except that it has three leads (emitter, base, and collector).
Figure 10 shows the equivalent circuit. The transistor conduction depends on the conduction of the photodiode. The base lead is seldom used. When it is used, its purpose is to adjust the turn-on point of the transistor.
Phototransistors can produce higher output current than photodiodes. Their response to light changes is not as fast as that of the photodiode. There is a sacrifice of speed for a higher output current.
Applications include photo-tachometers, photographic exposure controls, flame detectors, object counters, and mechanical positioners. Figure 11 shows the schematic symbol for a phototransistor. Figure 12 shows a typical circuit application.
Optical Coupler
Figure 13 shows an LED used in conjunction with a phototransistor to form an optical coupler. Both devices are housed in the same package. An optical coupler consists of an LED and a phototransistor. They are coupled by the light beam produced by the LED. The signal to the LED can vary, which in turn varies the amount of light available. The phototransistor converts the varying light back into electrical energy. An optical coupler allows one circuit to pass a signal to another circuit while providing a high degree of electrical insulation.
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